Carbon nanotube field-effect transistor with sidewall-protected metal contacts

ABSTRACT

A field effect transistor includes a substrate and a gate dielectric formed on the substrate. A channel material is formed on the dielectric layer. The channel material includes carbon nanotubes. A patterned resist layer has openings formed therein. Metal contacts are formed on the channel material in the openings in the patterned resist layer and over portions of the patterned resist layer to protect sidewalls of the metal contacts to prevent degradation of the metal contacts.

BACKGROUND Technical Field

The present invention relates to transistor devices, and moreparticularly to carbon nanotube transistors with protected metalcontacts.

Description of the Related Art

Among all the candidate materials for post-silicon era, carbon nanotubes(CNTs) have intrinsic ultra-thin body and exceptional electricalproperties (high on current, superior current on/off ratio, longmean-free-path for ballistic transport, etc.), which makes them one ofthe most promising candidates for sub-10 nm technology node insemiconductor industry. The implementation of CNT-based complementarymetal oxide semiconductor (CMOS) technology with low standby powerdissipation requires both robust high-performance n-type field effecttransistors (NFETs) and p-type field effect transistors (PFETs), whichare usually realized by using low- and high-work function metal contactsto the intrinsic (undoped) CNT channel, respectively.

While CNT p-type metal contacts (typically Pd) have been extensivelystudied and proven to be air-stable and robust, n-type metal contacts(typically Sc and Er) are usually prone to react with oxygen and hencesuffer from severe degradation over time through oxidation in air. Thisoxidation problem leads to low-yield and unstable NFETs in devicefabrication. To avoid the oxidation of CNT n-type metal contact, anotherlayer of metal (typically Au) is usually deposited on top to prevent theoxidation from the top surface. However, oxidation from the sidewallscould still dramatically degrade the contact properties.

SUMMARY

A field effect transistor includes a substrate and a gate dielectricformed on the substrate. A channel material is formed on the dielectriclayer. The channel material includes carbon nanotubes. A patternedresist layer has openings formed therein. Metal contacts are formed onthe channel material in the openings in the patterned resist layer andover portions of the patterned resist layer to protect sidewalls of themetal contacts to prevent degradation of the metal contacts.

A field effect transistor includes a semiconductor substrate configuredto form a gate electrode, a gate dielectric formed on the substrate andcarbon nanotubes formed on the dielectric layer to form a channel. Apatterned resist layer has openings formed therein, and n-type metalcontacts are formed on the carbon nanotubes in the openings in thepatterned resist layer. The metal contacts include extension regionswhich are formed over a portion of a top surface of the patterned resistlayer to protect sidewalls of the metal contacts to prevent degradationof the metal contacts. A dielectric material is patterned over the metalcontacts and the patterned resist layer to open contact windows to themetal contacts.

A method for forming a field effect transistor includes forming a gatedielectric on a semiconductor substrate; aligning carbon nanotubes onthe gate dielectric layer; patterning a first resist layer to formopenings therein; patterning a second resist layer on the first resistlayer; depositing metal on the carbon nanotubes in the openings throughthe first resist layer and over the first and second resist layers; andlifting off the second resist layer to form metal contacts where thesidewalls of the metal contacts are protected by the first resist layerto prevent degradation of the metal contacts.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view for a semiconductor device showing adielectric layer (gate dielectric) formed on a highly doped substrate(gate electrode) in accordance with the present principles;

FIG. 2 is a cross-sectional view for the semiconductor device of FIG. 1showing carbon nanotubes (CNT) aligned on the dielectric layer to form adevice channel for a field effect transistor (FET) device to be formedin accordance with the present principles;

FIG. 3 is a cross-sectional view for the semiconductor device of FIG. 2showing a patterned resist layer formed on the carbon nanotubes andhaving openings formed therein to locate metal contacts (source anddrain regions) in accordance with the present principles;

FIG. 4 is a cross-sectional view for the semiconductor device of FIG. 3showing a second resist patterned on the first resist in accordance withthe present principles;

FIG. 5 is a cross-sectional view for the semiconductor device of FIG. 4showing metal contacts formed through on the second resist layer using aliftoff process in accordance with the present principles;

FIG. 6 is a cross-sectional view for the semiconductor device of FIG. 5showing a dielectric layer formed over the metal contacts in accordancewith the present principles; and

FIG. 7 is a block/flow diagram showing methods for forming a CNT fieldeffect transistor in accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, devices and methods formaking the devices are provided to address the oxidation problem thatleads to low-yield and unstable n-type field effect transistors (NFETs)in device fabrication with carbon nanotubes (CNT). In usefulembodiments, a protective pattern forms metal contacts over itssidewalls to protect metal contacts for CNT NFETs from oxidation. Whiledifferent materials may be employed form the protective pattern, in oneembodiment, the protective pattern includes hydrogen silsesquioxane(HSQ), which can be reduced to silicon oxide after e-beam exposure. Anoverlapping portion of the metal contacts may is formed over theprotective pattern and employed to further protect the metal contacts tofrom oxidation.

While CNT p-type metal contacts include inert Pd, n-type metal contactsincluding Sc or Er react with oxygen and suffer from severe degradationleading to low-yield and unstable NFETs. In accordance with the presentprinciples, the oxidation problem is addressed using contact protectionpatterns resulting in increased yield and CNT NFETs with greaterstability.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer fox (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a partially fabricatedsemiconductor device 10 is shown in accordance with the presentprinciples. The device includes a substrate 12. The substrate 12 mayinclude a Si substrate; however, any suitable substrate may be employed.For example, the substrate 12 may include Si, SiGe, Ge, III-V materials,etc. A dielectric layer 14 is deposited on the substrate 12. Thedielectric layer 14 may include a grown oxide, a deposited oxide, adeposited nitride or other suitable dielectric material. If a depositedoxide is employed, the deposited oxide may include a high-k dielectricmaterial. Other dielectric materials may also be employed. Thedielectric layer 14 is employed as a gate dielectric for the substrate12, which functions as a gate electrode in the completed device. Thesubstrate 12 is highly doped to provide conditions for applying avoltage to carbon nanotubes (CNTs) as a gate electrode, as will bedescribed.

Referring to FIG. 2, highly purified CNTs 16 are aligned on thedielectric layer 14. The alignment of the CNTs may include placing theCNTs on the dielectric layer 14 or forming the CNTs on the dielectriclayer 14. Any suitable placement method or growth method may be employedto provide the CNTs 16. One approach to control the placement/alignmentof CNTs 16 is to control sites from which nanotubes are grown bypatterning catalyst particles on the dielectric layer 14. Patternedgrowth using a catalyst patterned on SiO₂/Si wafers can provide locationcontrol with chemical vapor deposition (CVD) growth of nanotubes fromthe catalyst islands.

Single catalyst-nanoparticle patterning may include the formation ofarrays of regularly spaced catalyst (e.g., Fe or Co) nanoparticles onSiO₂/Si (layer 14/substrate 12) by high-resolution electron-beam(e-beam) lithography (EBL). In other embodiments, formed CNTs 16 arestretched across the dielectric layer 16 as a CNTs sheet. The CNT sheetcan later be patterned as needed. Other suitable processes may beemployed to form CNTs 16 on the dielectric layer 14.

Referring to FIG. 3, a resist layer 18 is formed over the CNTs 16 andpatterned to form a pattern 20 for the formation of electrical contacts(to define a length L_(cont) for the contacts) for a field effecttransistor device to be formed using the CNTs 16 as a channel for thedevice. The pattern 20 also defines a channel length (L_(ch)). Theresist layer 18 may include a hydrogen silsesquioxane (HSQ) resist layer18. Other resist or dielectric materials may be employed instead of HSQ;however, HSQ is easily patterned using e-beam lithography, whichconverts the HSQ to SiO₂, which can be easily removed with known etchingtechniques. The resist layer 18 may be deposited by a spin coat processand patterned with e-beam lithography. The resist layer 18 is reduced toSiO₂ after e-beam exposure to form contact openings 21 (L_(cont)) afteran etched process. The openings 21 define locations for metal contacts,which will function as sources and drains for the CNT FET to be formed.

Referring to FIG. 4, a second resist material 23 is deposited (e.g.,spun on) and patterned over the resist layer 18. The pattern of thesecond resist layer 23 will be employed in a liftoff process for metalcontact formation. The second resist material 23 may include poly(methylmethacrylate) (PMMA) or other resists that may be employed in a liftoffprocess. The resist material 23 may be patterned using e-beamlithography. Since it is challenging to define a PMMA pattern withexactly the same openings as the underlying (HSQ) pattern, a smalloverlap (L_(ov)) may be defined to accommodate possible misalignmentwith the openings 21 in the resist layer 18.

Referring to FIG. 5, metal contacts 24 are formed in the openings 21(FIG. 4). The metal material for contacts 24 is formed by depositing thecontact material over the resist layer 23 (e.g., PMMA). The metaldeposition process may include any suitable deposition process, e.g.,sputtering, evaporation, CVD, etc. The metal material forms in theopenings 21, over the resist 23 and over the resist 18 in the overlapregions (L_(ov)).

Then, a liftoff process is performed to breakdown the resist 23 andremove the metal from over the resist 23. The metal remaining in theopenings 21 and the overlap regions (L_(ov)) forms the contacts 24. Themetal for the contacts 24 may include Er, Sc, La, Y, Ti or othersuitable n-type materials for NFETs. The contacts 24 function as sourceand drain regions for a field effect transistor (FET) device 30 formedwith the CNT 16 as the device channel.

In one embodiment, the contacts 24 may include a bilayer of metalmaterials. The bilayer may include an n-type material followed by aprotective material. For example, a Sc, Er, La, Y or Ti layer (n-typemetal) is formed followed by a layer of Au, Pt or other inert, highlyconductive material (protective metal). The inert, highly conductivematerial may be formed to protect a top surface of the contacts 24 whilethe resist 18 protects the sidewalls of the contacts 24. The inert,highly conductive material is deposited after the first metal for thecontacts 24 and is patterned using the same liftoff process. Here, inertparticularly refers to resistance to oxidation but protection from otherreactants is also included.

To further protect the contacts 24 from oxidation, the contacts 24extend over the resist 18 in the overlap regions (L_(ov)) to furtherprotect sidewalls of the contacts 24 (in contact with resist 18).Extension portions 26 of contacts 24 prevent oxidation of the contact ator near the CNT layer 16. Extension portions 26 also reduce contactresistance.

Referring to FIG. 6, a dielectric material 34 is formed over the device30. The dielectric layer 34 may be employed for forming vias and othermetallizations to connect to the contacts 24. The dielectric material 34may be formed using an atomic layer deposition (ALD) process, althoughother deposition processes may be employed. A pad open process may beemployed to form openings (not shown) through dielectric material 34 toaccess the contacts 24 in a trench 32 formed in the contacts 24 or onthe contact extensions 26. Additional metals may be formed to makeconnections to the contacts 24 through the openings (contact windows).In one embodiment, the dielectric material 34 includes Al₂O₃ depositedusing ALD and patterned using lithography to form contact windows.

The completed device 10 is preferably an NFET device since the n-typecontact metals for NFETs suffer more from oxidation than the metals(e.g., Pd) employed for PFET devices. However, the present principlesmay be employed for PFET devices as well. In particularly usefulembodiments, the channel length (L_(ch)) may be between about 200 nm and300 nm and nominally about 250 nm. The contact length (L_(cont)) may bebetween about 30 nm and 1200 nm. The overlap length (L_(ov)) may beabout 40 nm to about 60 nm, nominally about 50 nm. While the presentdimensions are provided, these dimensions should not be construed aslimiting as other dimensions may be employed in accordance with thepresent principles.

In tests performed in accordance with the present principles, yield wasmeasured for CNT NFETs with HSQ sidewall protection. The CNT NFETs withsidewall protection showed improvements in yield and reduced devicevariation. In one particular study, CNT NFETs with Sc contacts(L_(cont)=1000 nm) and sidewall protection (HSQ) had their yield closeto that of CNT PFETs with Pd contacts (L_(cont)=1000 nm) withoutsidewall protection. The NFETs with HSQ-protected Sc contacts(yield=95.2%) showed a higher yield than NFETs with Sc withoutHSQ-protected sidewalls (yield=82.1%).

In another study in accordance with the present principles, CNT NFETswith small Sc contacts (L_(cont)=40 nm) had an even more pronouncedyield improvement. The CNT NFETs with L_(cont)=40 nm and sidewallprotection (HSQ) were compared with CNT NFETs with L_(cont)=100 nmwithout sidewall protection (HSQ) (conventional structure). The NFETswith HSQ-protected Sc contacts (L_(cont)=40 nm) (yield=49.3%) showed ahigher yield than NFETs without HSQ-protected sidewalls (L_(cont)=100nm) (yield=14.0%).

Referring to FIG. 5, methods for forming field effect transistors areshown in accordance with the present principles. In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In block 102, a gate dielectric is formed on a semiconductor substrate.The gate dielectric layer may include a grown or deposited dielectricmaterial. The dielectric material may include an oxide, such as asilicon oxide or a high-k dielectric material such as, e.g., Al₂O₃.Other gate dielectrics may also be employed. The semiconductor substratemay include any suitable semiconductor material that can be doped toform a gate electrode. In some embodiments, the gate electrode can be apatterned metal electrode that has a top surface planarized for highspeed switching. The gate electrode (metal) would be formedcorresponding to the channel region (L_(ch)).

In block 104, carbon nanotubes are aligned on the dielectric layer. Thecarbon nanotubes may be placed on the dielectric layer or may be Rainedon the dielectric layer. The carbon nanotubes are aligned to form achannel region for a field effect transistor to be formed.

In block 106, a first resist layer is deposited (e.g., by a spin onprocess) on the carbon nanotubes. The first resist layer is patterned toform openings therein. The patterning of the first resist layer mayinclude electron beam lithography, although other patterning processesmay be employed. The patterned first resist layer may include hydrogensilsesquioxane (HSQ) or other resist material. The portions of the HSQto be removed maybe irradiated using an electron beam, which alters thematerial. The HSQ can be converted to silicon oxide (e.g., silicondioxide) by the electron beam. The altered material (silicon oxide) maythen be removed using an etching process to form the pattern.

The patterning of the first resist layer defines a channel length(L_(ch)) and contact lengths (L_(cont)) of the metal contacts by formingportions of the first resist layer to protect portions of the carbonnanotubes underlying the first resist layer.

In block 108, a second resist layer is formed and patterned on the firstresist layers. The second resist layer and the first resist layercombine to provide a patterned shape for forming source and draincontacts. The second resist pattern is offset from end portions of thefirst resist. The second resist layer is employed as a liftoff resist.

In block 110, metal contacts are deposited on the carbon nanotubes inthe openings in the patterned first resist layer and over the patternedfirst and second resist layers. A liftoff process is preformed to removethe second resist pattern to shapes the contacts. The contacts havesidewalls protected by the first resist layer and include contacts toprevent degradation of the metal contacts over the CNT in the openings.The liftoff process removes metal from regions where the second resistwas formed.

The metal contacts may an n-type metal to form the metal contacts. Inone embodiment, the resist for the liftoff process is formed. Then, then-type metal (e.g., Sc or Er) is deposited over the liftoff resist.Then, a protective metal (e.g., Au) is formed on the n-type metal. Then,in accordance with the liftoff process, the liftoff resist is removedforming the metal contacts.

In block 112, a dielectric material may be formed over the patternedmetal contacts. The dielectric material covers the metal contacts andmay be patterned to open contact windows. The dielectric layer may beformed using an ALD process although other formation processes may beemployed. The dielectric layer may be patterned using a pad open process(lithography).

In block 114, processing may continue to complete the transistor device.This may include forming interlevel dielectric layers, self-alignedcontacts, vias and other metallizations, etc.

Having described preferred embodiments for carbon nanotube field-effecttransistor with sidewall-protected metal contacts (which are intended tobe illustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

The invention claimed is:
 1. A method for forming a field effecttransistor, comprising: forming openings in a first resist layer overcarbon nanotubes; depositing metal on the carbon nanotubes in theopenings through the first resist layer and over the first resist layerand a second resist layer; and lifting off the second resist layer toform metal contacts where sidewalls of the metal contacts are protectedby the first resist layer to prevent degradation of the metal contacts.2. The method as recited in claim 1, wherein patterning the first resistlayer includes patterning the first resist layer using electron beamlithography.
 3. The method as recited in claim 1, wherein the firstresist layer includes hydrogen silsesquioxane (HSQ) and the methodfurther comprising etching the HSQ to convert the HSQ to silicon oxideby the electron beam.
 4. The method as recited in claim 1, whereinpatterning the second resist layer includes forming overlap regions byoffsetting the second resist from the openings in the first resist. 5.The method as recited in claim 1, wherein depositing the metal contactsincludes depositing a protective metal on an n-type metal to form themetal contacts.
 6. The method as recited in claim 5, wherein the n-typemetal includes Sc or Er and the protective material includes Au.
 7. Themethod as recited in claim 1, wherein depositing the metal includesforming extension contact regions over top portions of the first resistlayer.
 8. The method as recited in claim 1, wherein patterning the firstresist layer includes defining a channel length and contact lengths ofthe metal contacts by forming portions of the first resist layer toprotect portions of the carbon nanotubes underlying the first resistlayer.